The present invention relates to an underground drainage facility arranged such that influent fluids such as rainwater or the like flowing into water channels including small rivers is collected in an inflow channel and, more particularly, to a large capacity underground floodway with a slightly inclined inflow water channel which is provided in an underground location, with the floodway having a large depth of between 15 to 60 m, wherein the collected influent is lead to a drain pumping station, so as to be discharged into a river and the like where the influent is discharged and, with the underground drainage facility being suitable for reducing power consumption of drainage pumps, reducing volume or capacity of a pump well or time lag or delay in the following-up of flowing-down of the upstream influent at the start of the drainage pumps, and/or for measuring an upsurge at a suspension of operation of the pump. Recently, large underground drainage facilities have been proposed, wherein an inflow water channel having a large cross-sectional area is disposed at the deep location under the ground for a long distance. Underground facilities having a large depth in a solid or strong ground of a deep layer do not effect or influence an upper structure, and generally a depth equal to or greater than 50 m from the surface level is provided.
Conventionally, open channel flow operation drainage systems, wherein water flows down at a water level having a free surface in a conduit have been utilized, because inflowing water channels are not too deep and have a small distance, as compared with large underground drainage facilities of the type described hereinabove. However, closed channel flow operation wherein water flows down at a water level, in which the water fills the entire conduit have been proposed and the mixed flow operation of open channel and closed channels has also been considered. Either of these proposals however have a storage or reservoir tank for causing the same to have storage capabilities. If drainage pumps are installed in the open channel flow operation, pumps having a large capacity and pressure head are required, and the capacity of prime movers also increases. Furthermore, a valve on the discharge or delivery side is also required which has a large bore diameter and a high pressure, and a prime mover for opening and closing the valve also increases in size. If a large underground drainage facility is built in a conventional manner, in a relatively small underground drainage facility, not only do the constructional costs increase, but also there is an increase in the operating expenses. Thus, the conventional, relatively small underground drainage facility is uneconomical.
Moreover, it is necessary to provide a storage tank and to increase the capacity of the pump well because the storage advantages are reduced in the mixed flow operation of open channel and closed channel. This likewise increases construction cost.
Further, since the inflow water channel is lengthened considerably, water on the upstream side cannot be moved immediately following an operation of drainage pumps, even if the drainage pumps are operated. In a method in which drainage pumps are operated only in response to a level in a conventional pump well, there is a problem of delay or time lag in following flowing-down, which leads to an overflow of rivers or the like on the upstream side.
On the other hand, in the underground drainage facility, in order for a drainage of an estimated amount of water to be effected due to an abrupt increase in a quantity of inflow due to a rainfall a so-called waiting running is required in which running of drainage pumps starts before the influent reaches the pump well, that is, from a condition where a water level within the pump well is low. In a case of such waiting running, it is desired that a quantity of discharge of the drainage pumps is adjusted or regulated easily and economically in accordance with a change in a quantity of inflow. In order to satisfy such demand, a vertical-shaft adjustable vane pump in, for example, Japanese Patent Unexamined Application 57-1286092, is proposed in which a vane or blade angle of a pump impeller is variable.
Conventionally, a vertical-shaft adjustable-vane pump is used as a circulating pump, which circulating pump has a relatively low pressure head of, for example, equal to or less than about 30 m. However, a conventional vertical-shaft adjustable-vane pump is not used as a drainage pump which has a high pressure head such that the total pressure head reaches 50 to 60 m and which has a large capacity, for the following reasons.
When the total pressure head increases or becomes high, a fluid force applied to the adjustable vanes increases correspondingly. Accordingly, a load applied to a blade-angle operating mechanism which regulates a blade or vane angle of the impeller increases, so that the vane-angle operating mechanism increases in size. On the other hand, since there is a limitation in dimensioning, the blade-angle operating mechanism must be received in a limited space within a hub of the impeller, there is also a limit in an allowable load of the vane-angle operating mechanism. Accordingly, an adjustable vane pump cannot be applied to a drainage pump which is high in total head and which is large in capacity, as it is.
When a total pressure head increases, a specific speed Ns expressed by the following equation must be reduced to a value less than the conventional one. Accordingly, there is a problem that efficiency and suction performance are reduced. Specifically, in the following equation, the rotational speed N is determined by suction pressure of the pump and a cavitation condition, and a discharge Q and a pressure head H are given from the drainage plan. Accordingly, if the pressure head H increases, the specific speed Ns must necessarily be reduced to a low value. ##EQU1##
However, generally an adjustable impeller vane is arranged such that a shaft thereof is rotatably supported by a hub supported by a drive shaft, and is rotated about the axis of the shaft to adjust the vane angle. Accordingly, a tip surface on the side of a casing of the impeller and a hub surface on the side of a drive shaft must be formed respectively into concentric spherical surfaces to conform to an inner surface of the casing and an outer surface of the hub. In view of such restriction on shape, there is a problem that, if the specific speed is low, the efficiency and the suction performance are degraded.